Shaping of metal tubing



y 1961 F. c. DRAPER 2,992,473

SHAPING OF METAL TUBING Filed y 1956 4 Sheets-Shea; 1

Inventor Ff'fQ'GP/CA C. Draper July 18, 1961 F. c. DRAPER 2,992,473

SHAPING OF METAL TUBING Filed May 7, 1956 4 Sheets-Sheet 2 Ity A tiorneyy 1961 F. c. DRAPER 2,992,473

SHAPING OF METAL TUBING Filed May 7, 1956 4 Sheets-Sheet 3 Fi ll.

Inventor 47 2 4 7 Freder/offl aper Attorney 5' July 18, 1961 I F. c.DRAPER SHAPING 0F METAL TUBING 4 Sheets-Sheet 4 Filed May 7, 19562,992,473 SHAPING F METAL TUBING Frederick Cecil Draper, Sonorous Works,Deanbrook Road, Edgware, Middlesex, England Filed May 7, 1956, Ser. No.583,289 Claims priority, application Great Britain, May 11, 1955 6Claims. (Cl. 29-157) In general the Wall of a pipe or tube is too thinto, withstandsuch compressive stresses and unless precautions are takenwill crumple and cockle.

One method of counteracting this effect is to subject the tubing tointernal pressure so that before the bending takes place, the innersurface of the bend is in tension and consequently the superimposedcompressive stresses produced by the bending serve onh to cancel out theexisting tensile stresses. Thus the resultant stress is in the region ofzero and is not sufficient to produce cockling of the wall. As iswell-known, the longitudinal stress produced by internal pressure isonly half the circumferential or hoop stress so that when the internalpressure is increased to its maximum value, that is to say,corresponding to a value of hoop stress sufiicient to produce yieldingof the material, the longitudinal stress is only half this value. Intheory this should be suflicient to prevent cockling of the innersurface of the bend because if the neutral axis is symmetrically placedin relation to the section, the tensile stresses produced in the outersurface will be equal to the compressive stresses produced in the innersurface. Consequently when the existing tensile stress in the outersurface produced by the internal pressure is increased from half theyield value up to the full yield value by means of the applied bendingmoment, the existing tensile stress in the inner surface should bereduced by a corresponding amount to substantially Zero.

In practice, however, it is found that as soon as the outer surfacebegins to yield, the effect of work-hardening Is to raise the yieldpoint and consequently the neutral axis is no longer symmetricallyplaced, but moves outwardly of the section. Consequently a definitecompressive stress is produced in the inner surface. Provided the ratioof the diameter of the tubing of the wall thickness is fairly low, thewall is able to resist this compressive stress and an effective bend canbe produced. If, however, the ratio is larger, for instance, above about35:1 then the inner wall cockles and the process is no longer effective.

According to the present invention, a length of tubing to be bent issubjected to mechanically applied tension and is forced into contactwith a former having the required curvature in the plane of bending andshaped to support the tubing around approximately half its periphery.The tension is maintained at a value such as to avoid compressivestresses in the inner surface of the bend, which would produce cockling,and the outer surface of the bend is allowed tocollapse inwardly underthe tensile forces acting on it, after which the collapsed hen-t tubingis then expanded to its required final shape under the action of fluidpressure, while contained in a die.

At the start of the bending operation, the neutral axis is symmetricallyplaced in relation to the section of the tube, but as soon as thebending starts the tensile forces on the outer wall of the bend have aninward component whichtends to force the wall inwardly. This inwardforce is not resisted by internal pressure as in the process previouslyreferred to and the wall. accordingly starts to collapse. As aconsequence of the change of section, the neutral axis is movedinwardly. As the bending continues the process is progressive, the outerwall gradually collapsing further inwardly and the neutral axis alsomoving inwardly. Naturally the closer the neutral axis approaches to theinner surface, the less the compressive stresses set up by the bendingmoment and consequently these can be fully compensated for by themechanically applied tension so that the resultant stress in the innersurface of the bend is either tensile, or, if compressive, of such asmall value as not to be sufiicient to produce cockling.

It is found in practice that if the outer Wall is allowed to collapsefreely, that is to say, without the application of any internalpressure, it will move inwardly so as practically to come into contactwith the inner wall to give a section which is approximately U-shaped.The result obtained is thus practically the equivalent of that describedand claimed in the specification of Patent No. 2,748,455 except thatinstead of being brought about in a press as a preliminary step, thecollapsing of the tube occurs naturally, merely as a result of thebending operation. The final stage is the same in both processes in thatthe tubing is expanded to its required final shape under the action offluid pressure while contained in a die. The method in accordance withthe present invention, however, has the advantages firstly that thebending and the collapsing of the tubing is produced as the result of asingle operation and that in addition, if required, the former on whichthe bending is produced and which is shaped to support the inner surfaceof the bend, may constitute one half of the die used for the final stageof expansion. Thus once the tubing has been bent to shape on the former,it can be retained in position and parts constituting the remainder ofthe die may be brought up into register with the former so as toconstitute the complete die.

If, as will generally be the case, the tubing is circular incross-section, the surface of the former when seen in cross-section willbe semi-circular so as to support the inner surface of the bend. Duringthe collapse of the outer surface of the bend, the forces on the wallsof the tubing tend to distort the tubing laterally so that if thecross-section of the former were to stop short at the limit of thesemi-circle, the walls would tend to flow outwardly beyond the limit ofthe diameter. Preferably, therefore, the former is provided with partswhich form continuations of the cross-section, these continuationsextending either parallel to one another or diverging slightly to form alead-in. The presence of the continuation parts then constrains thetubing and prevents the lateral distortion referred to. If the former isalso to be used as half of the expansion die, then the continuationparts need to be formed as separate extension pieces which are removedbefore the remainder of the die is secured to the former.

In general a fixed former may be used and at the start of the bendingoperation the tubing is in engagement with the former at an intermediatepoint in its length, preferably its midpoint. The tubing is then forcedinto contact with the former along its length by appropriate movement ofits ends, while maintaining the necessary tension throughout. As analternative, however, the former may constitute part of a normal type ofbending capstan to which the tubing is secured at one end, the other endbeing allowed to yield as the capstan turns, while maintaining tension.The use of a capstan is satisfactory for angles of bend up toapproximately a rightangle, but if the angle is appreciably greater thanthis, the friction between the surfaces of the tubing and of the formercauses a progressive reduction of the tension along the length of thetubing. Thus although the tension may be at a sufficient value toprevent cockling at one end, it falls to a considerably lower value atthe other end and this lower value may not be sufiicient to preventcockling. Thus if a bend of the order of 180 degrees is required, whichis frequently the case, the previously mentioned method is preferred.The reason for this is that when the tubing is brought into contact withthe former at an intermediate point and then both ends of the tubing areforced into contact with the former, the efiect is the same as theproduction of two single bends, each of the order of ninety degrees. Inthis case the tension is a maximum at the two ends and decreases towardsthe middle, but does not reach a sufficiently low value for cockling toarise. This method has an additional advantage over the use of a capstaneven for small angles of bend in that curves other than circular arcscan readily be produced. Whatever the curvature of the former, provided,of course, that it is not re-entrant, the tubing is forced into contactwith it.

The invention will now be described in more detail by way of examplewith reference to the accompanying drawings, in which:

FIGURE 1 shows a length of tubing prior to bending;

FIGURE 2 is a diagrammatic illustration of the method of bending using afixed former;

FIGURE 3 is a view of the tubing of FIGURE 1 after the initial bendingbut prior to the final expansion;

FIGURES 4 and 5 show two stages in the bending of a length of tubing ona capstan;

FIGURE 6 is a sectional view of the bent tubing on the line VI-VI inFIGURE 5 FIGURE 7 shows a length of tapered tubing prior to bending;

FIGURE 8 shows the tubing of FIGURE 7 after bending and final expansionto shape;

FIGURES 9 and 10 show two stages in the operation of apparatus forcarrying out the method of FIGURES 1 to 3;

FIGURE 11 is a sectional view of the tubing prior to bending seen on theline XI--X[ of FIGURE 9;

FIGURE 12 is a sectional view of the tubing of FIG- URE 11 after theinitial bending operation; and

FIGURE 13 is a sectional view of the tubing after bending, seen on theline XIII-X'III in FIGURE 10.

Turning first to FIGURES l to 3, FIGURE 1 shows a length of parallelcircular tubing 1 prior to bending. This tubing has tension applied toit mechanically by means of rams acting in hydraulic cylinders showndiagrammatically as 2 and 3, the connections to the ends of the tubing 1being by means of collets 4 and 5 respectively. The tubing 1 is broughtinto contact at its midpoint with a former 6, which is on a fixedmounting. The former has the required curvature in the plane of bending,which in this case is a semi-circular arc. Moreover when seen incross-section, the surface of the former is also semi-circular so as tosupport the inner surface of the tubing 1 as it is bent into the dottedposition shown To achieve this result the hydraulic cylinders 2 and 3are moved in the direction of the arrows to the position shown in dottedlines, thus forcing the tubing 1 into contact with the former 6. Duringthis process the outer surface of the bend collapses inwardly as seen at7 in FIGURE 3 for the reasons previously described. The hydraulic ramsare allowed to float so that although they may be displaced within theirrespective cylinders, the tension in the tubing is maintained constantat a value which is suflicient to prevent the development of anyappreciable compressive stresses in the inner surface of the bend. Thecollapsed tubing of FIGURE 3 then merely requires to be expanded to itsfinal shape under the action of fluid pressure to give the finishedproduct.

It is found in general that if the tubing is fully annealed before thestart of the operation, it is possible to bend the tubing while allowingthe outer surface to collapse and then to expand the outer surface toits final shape in a single step without the need for any intermediatestages of annealing. By means of successive bending operations, compoundbends, that is to say, bends in more than one plane, may be produced inreadiness for the final step of expansion in a closed die. When a bendis required in the form of a circular arc of something less thandegrees, the method illustrated by FIGURES 4, 5 and 6, using a bendingcapstan may be employed. Here the tubing shown as 10 is secured at oneend by means of a collet 11 to a former 12 mounted on a capstan 13,while at the other end it is secured by means of a collet 14 to ahydraulic ram acting in a cylinder 15. As the capstan is rotated fromthe position of FIGURE 4 to that of FIGURE 5 so the hydraulic ram iswithdrawn from the cylinder 15 but the tension in the tubing ismaintained constant. During the bending operation the outer surface ofthe bend collapses inwardly from the position 16, seen in dotted linesin FIGURE 6, to the position 16. From this figure it will be seen thatthe outer surface of the bend is practically in contact with the innersurface and that the tubing at this stage is approximately U-shaped incross-section. The cross-section of the former 12 includes asemicircular portion 17 which supports the inner surface of the bend.Beyond the semi-circular portion there are continuations 18 shown whichdiverge slightly to provide a lead-in for the tubing at the formerturns. The main purpose of these continuations 18, however, is toprevent lateral distortion of the tubing beyond the limits of itsdiameter due to the outward components of the forces acting on it.

The tubing shown in the figures so far referred to is parallel, but themethod in accordance with the invention is particularly applicable tothe production of the various bends in wind instruments, and in generalthe tubing used for such bends is tapered as shown in FIGURES 7 and 8.FIGURE 7 shows the tubing 20 prior to bending, while FIGURE 8 shows thetubing after the formation of a bend of 180 degrees followed by theexpansion step to give the finished product. FIGURES 9 and 10 show inmore detail apparatus for bending the tubing of FIG- URES 7 and 8 inaccordance with the method of FIG- URES 1 to 3.

The apparatus includes a base 21 provided with a circular table 22. Thistable supports a die 23 which is shaped to produce the bend of FIGURE 8.The former 23 is carried by an hydraulic ram 24 working in a cylinder 25by means of which the former can be adjusted and can also be held inposition during the final stage of expansion. The tubing 20 is heldbetween collets 26 and 27 connected respectively to hydraulic ramsworking in cylinders 28 and 29. At the start of the bending operation,the cylinders 28 and 29 are situated diametrically opposite one anotherand apply the required degree of tension to the tube 20. With the tubingin this position the former 23 is moved into contact with the tubingunder the control of the hydraulic cylinder 25.

The two hydraulic cylinders 28 and 29 are mounted to move in arcs, thecentres of which lie in the region of the point of contact of the tube20 with the former 23. As shown diagrammatically in the drawing, thecylinders 28 and 29 are guided by means of pins 30 and 31 working inarcuate slots 32 and 33 in the table 22. The cylinders are driven aroundthe slots from the position of FIGURE 9 to that of FIGURE 10 by means offurther hydraulic cylinders 35 and 36 respectively. These are pivotedbeneath the base of the apparatus at 37 and 38 respectively and areconnected to their respective pins 30 and 3-1 by means of connectingrods 39 and 40.

Before the start of the bending operation, an extension piece 45 ismoved under the control of a further hydraulic cylinder 46 into theposition shown in FIGURE 9 where it cooperates with the former 23. Thepurpose of this extension piece 45 is to provide continuations, shown as47 in FIGURE 11, of the cross-section of the former so as to preventlateral distortion of the tubingas previously described. With theextension piece 45 in position, pressure is then applied to thehydraulic cylinders =35 and 36 to drive the cylinders 28 and 29 alongtheir slots 32 and 33 so as to force the tubing 20 into contact with theformer as shown in FIGURE 10. During this operation the two cylinders 35and 36- rock about their respective pivots and as the cylinders 28 and29 approach the end of their travel, the cylinders 35 and 36 rock backagain until, as seen in FIGURE 10, they are in approximately the sameangular position as in FIGURE 9, although, of course, the connectingrods 39 and 40 have been withdrawn into their respective cylinders.

During the bending operation, the outer wall of the tubing is allowed tocollapse inwardly and it finally reaches the position shown in FIGURE12. When the bending is complete, the extension piece 45 is withdrawnunder the control of the cylinder 25 and is replaced by parts 50 and 51,which are moved inwardly under the control of hydraulic cylinders 52 and53 respectively from the position of FIGURE 9 to that of FIGURE 10.These two pieces, together with the former 23, constitute a complete diefor the final expansion of the tubing. The cross-section of this die isseen in FIGURE 13, from which it is clear that the part 5 1 fits intothe space previously occupied by the extension piece 45 to constitute adie having an internal space of circular cross-section. Since this dieis required to resist considerable internal pressure, it is importantthat the parts should be held securely in position and the angles of thethree hydraulic cylinders 25, 52 and 53 acting at substantially 120degrees to one another ensure that the considerable forces exerted bythese cylinders are in equilibrium.

The pressure in the cylinders 28 and 29 is released to relieve thetension in the tubing 20, but the two cylin ders still act to hold theinner surface of the bend firmly in contact with the correspondingsurface of the die to prevent any risk of cockling during the subsequentexpansion. Hydraulic pressure is then applied to the interior of thetubing 20 by means of a connection 55 leading to the collet 26. Theeifect of this internal pressure is to expand the tubing outwardly fromthe position shown in full lines in FIGURE 13 to that shown in dottedlines, and after this the parts of the die 50 and 51 may be withdrawnand the collets 26 and 27 disconnected to leave the finished product ofFIGURE 8.

The hydraulic pressure required for the final expansion of the tubing,which is supplied to the connection 55, can be drawn from the samesource of pressure as supplies the various hydraulic cylinders. Thesupply to the cylinders 28 and 29 is shown diagrammatically by way ofthe connections 56 and 57, but for simplicity the hydraulic connectionsto the other cylinders are not shown. It will be understood, however,that once the tube has been secured in position between the collets 26and 27, the whole apparatus may be controlled by means of the variousvalves controlling the hydraulic pressure to the various components andthat the com- 6 plete bending operation can be carried out in a veryshort space of time by means of an extremely simply sequence of steps bythe operator.

I claim:

1. A method of forming a bend in a length of ductile metal tubing ofsubstantially circular cross-section, comprising the steps of subjectingsaid tubing to mechanically applied tension, bringing said tubing intocontact with a former, said former having the required curvature in theplane in which said tubing is to be bent and being shaped to supportsaid tubing around approximately half its periphery, bending said tubingabout said former while maintaining the tension in said tubing at avalue which avoids compressive stresses in the inner surface of the bendin said tubing sufficient to produce cockling, but which causes theouter surface of the bend to collapse inwardly under the tensile forcesacting on it and expanding the collapsed bent tubing to its requiredfinal shape under the action of fluid pressure while contained in a die.

2. A method according to claim 1, in which the former is employed aspart of the die used for the final expansion.

3. A method according to claim 1, in which lateral distortion of thetubing beyond the limits of its diameter during bending is prevented byexteriorly supporting the lateral portions thereof as its inner surfaceapproaches said former.

4. A method according to claim 3, in which the exterior support for thelateral portions of the tubing is withdrawn before the die is secured tothe former before the final expansion.

5. A method according to claim 1, in which during the bending operationthe former is fixed and the tubing is initially in engagement with theformer at an intermediate point on its length, the tubing then beingforced into contact with the former along its length by appropriatemovement of its ends, while maintaining the tension.

6. A method according to claim 1 in which one end of the tubing isattached to said former and said former is turned while the other end ofsaid tubing is yieldingly held with a force sufficient to maintain saidtension.

References Cited in the file of this patent UNITED STATES PATENTS301,106 Follett July 1, 1884 757,592 Atwood Apr. 19, 1904 951,717 AndresMar. 8, 1910 2,233,556 Rieger Mar. 4, 1941 2,357,027 Seifried Aug. 29,1944 2,457,483 Martin Dec. 28, 1948 2,459,132 Neilsen Ian. 11, 19492,476,556 Maize July 19, 1949 2,514,831 Bath July 11, 1950 2,536,738Green Jan. 2, 1951 2,746,727 Earl May 22, 1956 2,748,455 Draper et a1.June 5, 1956 2,806,506 Yurka Sept. 17, 1957 2,837,810 Ekholm June 10,1958

